1. Field of the Invention
This invention relates generally to the field of modems for Digital Subscriber Line (DSL) applications. More particularly, the invention provides a hybrid architecture modem for use in Time Division Multiplexed (TDM) DSL which eliminates the requirement for a balanced network.
2. Description of Related Art
Most xDSL systems operate with Frequency Domain Duplex (FDD) or Frequency Domain Multiplex (FDM) signal transmission. The transmit and receive signals for these systems travel concurrently over the same twisted pair cabling. To be able to receive the incoming signal while transmitting the outgoing signal, modems typically employ a trans-hybrid balanced network. In such a modem, the line driver serves as both the source for the transmitting signal and the terminator for the receive signal.
FIG. 1 shows a typical prior art device. Resistors 2 and 4, having resistance values R1 and R2 respectively, are the loading resistors for the twisted pair line 6 and are terminated by the line driver 8. Inductors 10, 12, 14 and 16 having impedances Za, Zb, Zc, and Zd form the hybrid balanced network (HBN) that separates the receive signal from the mixed signals on the twisted pair. The HBN not only separates the receive and transmit signals, but also helps eliminate the appearance of noise from the line driver on the receive output. Noise created by the line driver is usually random in nature and could lower the receiver's signal to noise ratio (S/N) if not suppressed. Typically, the HBN can attenuate the transmit signal and noise by 20 dB on the receive output lines 18; enough to eliminate the effect of line driver noise on the receive S/N.
The penalty of using the HBN is that the receive signal is also attenuated by about 3.5 dB. This degrades the receive signal's noise floor and makes the design of the receiver more difficult. Another side effect of the HBN is that it increases the line driver's power dissipation. A trade-off exists between thermal noise and the extra power dissipation. A ten percent increase in line driver power consumption is not uncommon due to the HBN.
In many modems, so called “active termination” schemes are used in the hybrid circuit to increase power efficiency. These schemes use active impedance to replace a portion of the line loading resistor so that smaller resistance values can be used, resulting in reduced waste in transmitter power. A typical actively terminated DSL modem hybrid circuit is shown in FIG. 2.
Active termination is created by the addition of resistors 20, 22, 24 and 26 having resistances R3, R4, R5 and R6, which provide feedback from the HBN nodes to the line driver. The degree of active impedance synthesis can be measured by the active termination ratio (ATR) defined as (R1+R2)/Zo where Zo is the nominal line impedance. In most DSL loops Zo=100 ohms. Conventionally R1=R2=Zo/2 is required to properly terminate the line. In an active termination hybrid R1=R2<Zo to reduce line driver power consumption. In the extreme case of R1=R2=0, 50% of the line driver power dissipation is saved. In practical embodiments, the ATR ranges from 0.1 to 0.5.
However, introduction of active termination further attenuates the receive signal in the modem. This attenuation is roughly proportional to the ATR. An ATR of 0.2 saves power by 40% but also results in an additional 14 dB attenuation of the receive signal, making the total noise floor 17.5 dB worse. The DSL line background noise is −140 dBm. The exemplary active impedance modem changes the effective noise floor to −157.5 dBm, making the receive amplifier design difficult and costly.
It is therefore desirable to employ a modem architecture for DSL, which does not include a HBN, to reduce the noise floor degradation to simplify amplifier design requirements for the receiver.